This research takes advantage of the unique opportunity presented by the accurate measurement of percent body fat using Dual-emission X-ray absorptiometry (DXA) in the nationally representative 1999- 2004 National Health and Nutrition Examination Survey (NHANES). This will be the first study to use the 10 anthropometric, 2 bioelectric impedance (BIA) and demographic variables collected to produce equations that will enable investigators to calculate valid measurements of percent body fat in children and adults using variables that are feasible to collect in a wide range of field settings. Most previous equations developed to predict percent body fat using anthropometrics and BIA were calculated using smaller samples assembled by convenience and often very limited in race/ethnic and age diversity. Our specific aims are: Aim 1: Create and validate gender-specific equations for youth and adults to estimate percent body fat in a diverse, nationally representative sample. Aim 2: Generate equations with successively smaller numbers of variables than the optimal equations. These equations will anticipate the needs of researchers who have available or have the opportunity to collect only a limited number of variables. Aim 3: Generate equations that exclude either the two BIA variables or the two skinfold variables. Since performance of BIA and collection of skinfolds are more costly in terms of equipment and training than other measures, we will provide equations from which they are excluded. Aim 4: Disseminate the developed percent body fat equations through publications and a website and application software (app) that makes them readily available and easily useful to researchers, clinicians and the public. This work uses a well-qualified team of investigators to leverage the value of extant data from NHANES to produce percent body fat equations that are generalizable to American children and adults.